The present invention relates to a process for controlling the isomer distribution of the chlorination products obtained in the liquid phase chlorination of 2-chloro-6-(trichloromethyl)pyridine by regulating the concentration of hydrogen chloride in the system.
The chlorination of 2-chloro-6-(trichloromethyl)pyridine in the liquid phase at elevated temperatures is disclosed in U.S. Pat. No. 4,256,894 to produce 3,6-dichloro-2-(trichloromethyl)pyridine and 5,6-dichloro-2-(trichloromethyl)pyridine as well as higher chlorination products, such as 2,3,4,5-tetrachloro-6-(trichloromethyl)pyridine, 2,3,5,6-tetrachloropyridine, and pentachloropyridine. Hydrogen chloride is a by-product. Catalysts, such as metal halides, especially ferric chloride, are often employed. Application of this process to the preparation of either 3,6-dichloro-2-(trichloromethyl)pyridine or 5,6-di-chloro-2-(trichloromethyl)pyridine specifically is deficient in that both of these two monochlorination products are always formed and there is no way reported to alter their ratio in favor of the isomer specifically desired.
Since the dichloro-6-(trichloromethyl)pyridine isomers are much more valuable as individual entities than as mixtures and because typically only one of the isomers is desired at any one time, methods of controlling the isomer distribution obtained in the chlorination are highly desirable. 3,6-Dichloro-2-(trichloromethyl)pyridine is useful as an intermediate in the production of 3,6-dichloropicolinic acid, a commercial herbicide, and other valuable compounds. 5,6-Dichloro-2-(trichloromethyl)pyridine is similarly useful as a chemical intermediate for agricultural chemicals and pharmaceuticals.